The present invention relates to a method for reducing residual volatiles from polymer compositions.
Polymer compositions normally contain unwanted volatile organic constituents, such as residual monomers from incomplete conversion, impurities in the starting materials, initiator decomposition products, products of low molecular weight from secondary reactions, etc. These components are generally referred to as residual volatiles or VOCs (volatile organic components).
Basically, there are two methods of removing residual volatiles: chemical and physical deodorization. In chemical deodorization, a post-polymerization converts extant polymerizable components (primarily the monomers employed) into oligomers and polymers which are no longer volatile. Thus, e.g. in the preparation of polymer solutions or polymer dispersions from ethylenically unsaturated monomers, post-polymerization usually involves an initiator different from that, similar to that or the same as that used in the main polymerization.
Chemical deodorization has the drawback that only the polymerizable residual volatiles can be eliminated. All saturated secondary components from impurities of the starting materials and secondary reactions of the additional ingredients remain as residual volatiles in the product. At the same time, chemical deodorization itself may also lead to the formation of secondary components. As an example, acetone and t-butanol may be formed when t-butyl hydroperoxide/acetone bisulfite are used as redox-initiator system in the polymerization, and formaldehyde and t-butanol may be formed when t-butyl hydroperoxide/sodium hydroxymethanesulfinate are used as initiators. The deodorization of polymers on the basis of esters of vinyl alcohol with C1-C30-mono-carboxylic acids may lead to ester cleavage and secondary products. Thus, the deodorization of polymers on the basis of vinyl acetate may lead to the formation of acetic acid or acetaldehyde. Both the impurities and the newly introduced substances remain in the polymer product as residual volatiles.
In the case of physical deodorization, the liquid polymer composition is stripped with a stripping agent, e.g. steam, in a stirred vessel. A problem occurring in continuous stripping of liquid polymer compositions and especially of polymer dispersions containing or acting as emulsifiers is the formation of foam.
DE 12 48 943 teaches a process for the production of odour-lean aqueous polymer dispersions by leading steam or hot inert gas through the boiling dispersion and repeated short decreases in pressure by 100 to 350 Torr to remove the foam.
DE 196 21 027 A1 and DE 197 16 373 A1 describe a physical deodorization process for the continuous removal of monomers from aqueous suspensions or dispersions using steam in a counter-current flow column fitted with sieves.
Physical and chemical deodorization may also be employed simultaneously, as is described in U.S. Pat. No. 4,529,753. This document describes a process wherein a polymer dispersion is subjected to temperature and pressure conditions at which the vapour pressure of water in the ambient atmosphere is lower than the vapour pressure of water in the dispersion. The emulsion is not significantly degraded or destabilized and into the emulsion a free radical generator is introduced until the residual monomer content is not more than 0.05 percent by weight.
EP 0 967 232 A1 relates to a process for removing residual volatiles from polymer dispersions by performing first chemical deodorization to remove residual monomers and then physical deodorization to remove volatile components.
EP 0 650 977 A1 describes a method of removing residual monomers from polymer dispersions which involves sequential application of chemical and physical deodorization. In comparison to the process of U.S. Pat. No. 4,529,753, the method of EP 0 650 977 A1 is assumed to result in lower residual monomer concentrations within an acceptable time frame. To achieve this, the dispersion is first treated with a sufficient amount of an initiator system until the residual monomer content is reduced to 1500 to 6000 ppm. Then follows a physical deodorization with steam under subatmospheric pressure, in the course of which the residual monomer content is reduced to about 5 to 500 ppm.
In a first preferred embodiment of EP 0 650 977 A1, precisely controlled vacuum is applied to the tank vessel. The vacuum is set at a selected value of absolute pressure which will determine the boiling point of the water in the system and hence the desired temperature at which the stripping operation is to be carried out. Typically, the procedure is carried out at the absolute pressure corresponding to the boiling point of water at the particular temperature. When the desired pressure is achieved, controlled injection of live steam is started at the bottom of the vessel to allow contact of the steam with the emulsion as the steam passes through the liquid upwards. The agitator runs throughout the process. At the beginning of the steam injection process, the temperature of the latex is lower than the boiling point. Hence, during this phase, the steam condenses in the latex releasing its latent heat and serves to elevate the batch temperature. When the batch temperature reaches the boiling point of water at the set pressure, no further rise in temperature is evidenced, and the actual stripping is taking place. The vapors drawn off from the process are directed by a pipework at the top of the vessel to the tube side of a shell-and-tube type condenser or can be removed by other means, for example, by means of a jet eductor. The vapors condense and the resulting liquid drains to a receiver vessel for later disposal. Steam injection continues until the desired level of residual monomer is achieved. Then the steam injection is stopped and the pressure is lowered to full vacuum.
In a second (not preferred) embodiment of EP 0 650 977 A1, the polymer composition is transferred to the stripping vessel and vacuum is applied to the vessel in an uncontrolled fashion. In this embodiment, steam is injected not at a constant rate, but at a rate so as to maintain a constant temperature in the stripping vessel. It is a disadvantage of this embodiment that during the initial application of vacuum a great deal of foam is generated and, depending on type of product, may fill the vessel and the overheads (pipework and condenser) with foam.
There is still a need for an effective method for reducing residual volatiles from polymer compositions that are essentially free from water. This regards in particular polymer compositions that are surface active and tend to a formation of foam. A special problem is the removal of unwanted volatile organic constituents from polymer compositions that are water-sensitive, e.g. because they might undergo a hydrolysis.